The present invention relates to a process for preparing an N,N-dialkylethanolamine having high color stability.
N,N-dialkylethanolamines are important intermediates for the chemical and pharmaceutical industry. Dimethylethanolamine finds use in a wide variety of different fields, for example in the form of salts, soaps, ethers and esters, as an emulsifier and surface-active substance and as a catalyst in polyurethane chemistry. In the pharmaceutical industry, it is used to synthesize active ingredients (tranquilizers, antihistamines and analgesics). Discoloration in N,N-dialkylethanolamines, such as dimethylethanolamine, is undesirable in most applications.
The addition of almost all amines onto ethylene oxide (EO) is, like that of ammonia, accelerated considerably by addition of water. For instance, heating to 150° C. for several hours is necessary for reaction of EO with dimethylamine (DMA); in contrast, the reaction proceeds even under cold conditions when an aqueous DMA solution is used. An effect similar to water is also possessed by alcohols such as methanol or ethanol (Houben Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Volume 11/1, 1957, page 311 ff.).
It is known that alkanolamines react further as a result of ethoxylation of the hydroxyl group to give more highly ethoxylated products. This further reaction can be substantially suppressed by use of excess amine (1.1:1 to 4:1) (DE 23 57 076 A, DD 203 534 A (VEB Synthesewerk Schwarzheide), U.S. Pat. Nos. 2,337,004 A, 2,373,199 A).
It is also known that tertiary amines, for example dimethylethanolamine (DMEOA), react both in the absence and in the presence of water below 80° C. with oxiranes to give thermally labile quaternary ammonium compounds which decompose more or less rapidly above 90° C. (E. Tobler et al., Helv. Chim Acta 52, 1969, page 408-418). The formation of these bases is disadvantageous in several ways:
1) Fixing of EO in the quaternary bases leads to selectivity losses.
2) Due to their high basicity, the quaternary bases catalyze the formation of more highly ethoxylated compounds. Dodecylamine undergoes addition, for example, of 2 mol of EO above 200° C. (→no quaternary bases exist) and of 10 mol of EO below 90° C. (→quaternary bases exist) to give more highly ethoxylated compounds (H. L. Sanders et al., J. Am. Oil Chemists' Soc. 46, page 167-170). The overall result is selectivity losses.
3) It is assumed that the undesirable formation of discoloration in the DMEOA is attributable to complicated decomposition mechanisms (E. Tobler et al., Helv. Chim. Acta 52, 1969, page 408-418) of these quaternary ammonium compounds to give volatile unsaturated compounds, which then polymerize (DD 203 534 A).
What follows is prior art with regard to the preparation of color-stable dialkylethanolamines.
Water-catalyzed processes with specific workup:
EP 70 978 A (Pennwalt Corp.): Continuous reaction of excess DMA (2.2 eq) with EO in the presence of water (0.2-0.5 eq) at 150°; distillative workup with addition of defined amounts of sodium borohydride.
U.S. Pat. No. 3,131,132 A (Jefferson Chem. Comp., Inc.): Batchwise reaction of excess DMA (1-2 eq) with EO in the presence of water (3-15 eq) at 50-100° C.; distillative workup (190 mbar) after adjustment of the pH to 11.5 by addition of acid.
JP 01160947 A (Mitsubishi Gas Chem. Co., Inc.): Water-catalyzed synthesis of DMEOA; workup by distillative high boiler removal (100 mbar); hydrogenation of the distillate over Ru/C; purifying distillation at 100 mbar.
Disadvantages of these processes are the additional material and energy costs resulting from the aftertreatment.
Specific workup or aftertreatment variants:
JP 61186349 A (Daicel Chem. Ind., Ltd.): Distillative workup (200 mbar) of crude DMEOA with addition of urea.
JP 57021351 A (Mitsubishi Gas Chem. Co., Inc.): Hydrogenation of crude DMEOA over Ru/Al2O3.
EP 28 555 A1 (PCUK Produits Chim. Ugine Kuhlmann): Hydrogenation of crude DMEOA over Raney nickel.
DE 30 40 732 A1 (PCUK Produits Chim. Ugine Kuhlmann): Purification of DMEOA by after-treatment with ammonium salts.
U.S. Pat. No. 6,774,264 B2 (Air Prod. and Chem., Inc.): Hydrogenation of crude DMEOA Pd/Al2O3.
Disadvantages of these processes too are the additional material and energy costs resulting from the aftertreatment.
Anhydrous processes (T<200° C.):
JP 01157938 A (Mitsubishi Gas Chem. Co., Inc.): Continuous process at 150° C./21 bar; recycling of defined amounts of the reaction output into the reactor (autocatalysis).
DE 23 57 076 A1 (BASF AG): Continuous process; recycling of defined amounts (0.01-0.5 times the amount based on EO plus DMA) of the reactor output (autocatalysis). Color-stable products in reactions <160° C.
Processes with stabilizing additives:
U.S. Pat. No. 3,567,779 A (Jefferson Chem. Comp., Inc.): Inhibition of discoloration by addition of mono- or diethanolamine to give DMEOA.
A disadvantage is that stabilizing additives are undesirable for many applications of N,N-dialkylethanolamines such as DMEOA.
Water-catalyzed processes without aftertreatment:
DD 203 534 A: Reaction of excess DMA (1.1-3.5:1) with EO in the presence of catalytic amounts of water (0.02-0.15 eq) under very mild reaction conditions (50-90° C.); distillative workup with max. still temperature of 90° C.
A disadvantage is that the lowering of the reaction temperature below 90° C. according to the literature (Helv. Chim. Acta 52, 1969, page 408-418) leads to the formation of quaternary bases (→yield losses). Quaternary bases can release acetaldehyde as a result of thermal decomposition in the subsequent workup and lead to poor color stability in the end product.
EP 752 412 A1 (BASF AG) (equivalent DE 44 14 879 A): Reaction of excess DMA with EO in the presence of water (2.5-50% by weight, preferably 8-25% by weight) at reaction temperatures of ≧90° C. and distillative workup at temperatures of max. 90° C.
A disadvantage is that the bottom temperature in the distillative workup has to be kept 90° C.
Water-catalyzed reactions of ethylene oxide with dimethylamine at reaction temperatures of >90° C. are prior art (e.g. EP 70 978 A; DE 44 14 879 A, see above). The color stability of DMEOA produced in such a way, however, has to be ensured either by subsequent treatment of the crude output (for example with a reducing agent/post-hydrogenation (U.S. Pat. No. 6,774,264, see above) or with acids) or by maintaining maximum bottom temperatures (<90° C., DE 44 14 879 A) in the purifying distillation.